Date: Thu, 30 Oct 2003 01:20:17 +1030 From: Malcolm Kay <malcolm.kay@internode.on.net> To: "nw1" <network101@covad.net>, <freebsd-questions@freebsd.org> Subject: Re: Drive Geometry -- confusion. Message-ID: <200310300120.17617.malcolm.kay@internode.on.net> In-Reply-To: <001101c39e12$dac4b3f0$0300a8c0@install> References: <001101c39e12$dac4b3f0$0300a8c0@install>
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On Wed, 29 Oct 2003 22:20, nw1 wrote: > FreeBSD 4.8-RELEASE-p13 > This document can also be viewed @ > https://69.3.136.141/freebsd/installation/geometry > > Install 4.8 from CD-rom > > Drive Geometry confusion. > My BIOS allows me a very limited amount of settings when it comes to an > HDD. The following are the only settings offered by my BIOS: > > Read Prefetch - [Disabled | Enabled] - set to (Disabled) > Disk Bios Translation - [LBA | CHS | Disabled] - set to (LBA) > Local Bus IDE Mode - [High Performance | Compatible] - set to (High > Performance) > > Size - 8455 MB --Auto-detected-- This is actually an 80GB HDD model: > WD800JB. (Unable to change the 'Size' setting.) > > The only other setting that could remotely be associated with any HDD's > installed would be the 'boot order' of the devices. > > ... sysinstall main menu | Custom | Partition : > > Here's where I'm confused; The following dialog message is printed: > > WARNING: A geometry of 155061/16/63 for ad0 is incorrect. Using a more > likely geometry. If this geometry is incorrect or you are unsure as to > whether or not it's correct, Please consult the Hardware Guide in the > Documentation submenu or use the (G)eometry command to change it now. > > Remember: you need to enter whatever your BIOS thinks the geometry is! = For > IDE, it's what you were told in the BIOS setup. For SCSI, its the > translation mode your controller is using. Do NOT use a "physical > Geometry". > > ## With the above two (2) paragraphs having been said, All I have at t= his > point is the physical geometry given by the WD800JB specs --which the a= bove > paragraphs are currently fobbing me to use. > > Lets press <enter> for OK and move further into this... > > We're in the FDISK Partition Editor where it states on the second line = from > the top: DISK Geometry: 9729 cyls/255 heads/63 sectors =3D 156296385 se= ctors > (76316MB) > > If I understand correctly, the physical geometry of this WD800JB is: > > Cylinders =3D 16383 > Heads =3D 16 > Sectors/Track =3D 63 A simple calculation from these figures gives a total capacity of about 8= =2E4Gb. In no way are these figures real (or even equivalent) for an 80Gb drive. = The=20 problem is that the hardware/software does not have the bit capacity to=20 recognise more than 16383 cylinders. The 80Gb corresponds roughly with th= e=20 155061/16/63 figure. In fact modern drives at the disk hardware level expect simple absolute s= ector=20 number for addressing not CHS, and in fact there may be different numbers= of=20 sectors to a track dependent on its diameter. So the 155061/16/63 figures= are=20 only some sort of equivalent CHS used in translating CHS addresses to=20 absolute sector numbers. In principle you could pretend the CHS geometry = was=20 almost anything you want so long as it allows translation of hypothetical= CHS=20 addreses to real absolute sector addresses. The catch comes in the limitations of the bios/software/hardware register= s to=20 hold the CHS quantities. Using maxima for heads (255) and sectors (63) ke= eps=20 the number of cylinders to a realisable value. Hence 9729/255/63 looks li= ke=20 the right choice. Malcolm Kay
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